Dehydrogenation

Abstract: The dehydrogenation of an alkane to an alkene, especially isobutane to isobutene, is carried out in admixture with oxygen and in the absence of added steam over a dehydrogenation and oxidation catalyst comprising a platinum group metal deposited upon a support. A catalyst comprising platinum deposited on a support which is a mixture of tin oxide and zirconium oxide possesses good activity for the dehydrogenation of an alkane, especially carried out in this way.

Description: The present invention concerns a process for the dehydrogenation of an alkane to form an alkene. It is known to dehydrogenate isobutane to isobutene using direct dehydrogenation at low space velocity (GHSV=100-1000hr -1 ). The conventional industrial process has several inherent disadvantages: (a) it is an endothermic reaction, requiring high thermal input; (b) the yield of isobutene is equilibrium limited; and (c) at temperatures favouring high yields of isobutene, the rate of catalyst de-activation is also high. Improvements to the conventional process have included the addition of either steam ( eg USP 4,926,005 and 4788371) or hydrogen ( eg USP 4,032,589) to the gas feed. The function of the hydrogen is as a diluent, and to reduce the deposition of carbon on the catalyst. The steam improves thermal conduction through the catalyst bed and reduces the deposition of carbon on the catalyst, and hence it too has been used as a diluent. The catalysts used in industry include platinum on alumina, platinum on tin oxide and chromium oxide-based catalysts. There remains a need for an improved process for the dehydrogenation of alkanes, especially for the dehydrogenation of isobutane, which is a starting material for MTBE (methyl-tert-butyl-ether) production. The conventional processes require high inputs of energy and the capital cost of a catalytic reactor designed to supply large amounts of heat is particularly high. Moreover, conventional processes demonstrate rapid catalyst deactivation, so that expensive and complex catalyst regeneration has to be designed into the equipment and the process. The present invention provides an improved process for alkane dehydrogenation. Accordingly, the invention provides a process for the dehydrogenation of an alkane to form an alkene, comprising passing a feedstock comprising said alkane in the gas phase in admixture with oxygen and in the absence of added steam over a dehydration and oxidation catalyst comprising a platinum group metal deposited upon a support. The present process is advantageous over the known processes by reason of one or more of such features as higher yield of the alkene, higher selectivity to the alkene, lower operating temperature, lower heat input, a simpler system and lower catalyst deactivation. There is much prior art on the dehydrogenation of alkanes to alkenes, (though a scant amount on the oxidative dehydrogenation of alkanes to alkenes), yet the present improvements were not realised before. As explained in the US specification 4788371 mentioned above, the dehydrogenation of hydrocarbons is endothermic. In a system employing a dehydrogenation catalyst only, it is typically necessary to add superheated steam at various points in the process or to intermittently remove and reheat the reaction stream between catalyst beds. In an improvement, processes were developed which utilised a two-catalyst system with distinct beds or reactors of dehydrogenation or selective oxidation catalysts. The purpose of the selective oxidation catalysts was to selectively oxidise the hydrogen produced as a result of the dehydrogenation with oxygen that had been added to the oxidation zone to generate heat internally in the process. The heat generated would typically be sufficient to cause the reaction mixture to reach desired dehydrogenation temperatures for the next dehydrogenation step. The US specification explains that in its invention one specific catalyst can be used to accomplish both the dehydrogenation and oxidation reactions. It discloses a process for the steam dehydrogenation of a dehydrogenatable hydrocarbon with oxidative reheating which comprises contacting a dehydrogenatable hydrocarbon comprising C 2 -C 15 paraffins and steam at a steam to hydrocarbon molar ratio of from 0.1:1 to 40:1, at a pressure from 0.1 to 10 atmospheres, a temperature of from 400 to 900°C, and a liquid hourly space velocity of from 0.1 to 100hr -1 with a catalyst in the first reaction zone of a reactor containing a plurality of reaction zones and introducing an oxygen-containing gas into the second, and every other reaction zone of the plurality of reaction zones such that the total rate of the oxygen-containing gas introduced into the reaction zone ranges from 0.01 to 2 moles of oxygen per mole of C 2 -C 15 paraffin feed wherein the catalyst is comprised of from 0.1 to 5 weight% platinum, and from 0.01 to 5 weight% potassium or cesium or mixtures thereof on an alumina support having a surface area of from 5 to 120m 2 /g and recovering the products of the reaction. Though the specification mentions the possibility of a single reaction zone within a single reactor with single inlet and outlet parts, all co-feeds entering the inlet of the reactor and products and by-products leaving the system through the reactor outlet part, there is no Example illustrating this concept. Moreover, the present broad process involving passing an alkane in admixture with oxygen over a dehydrogenation and oxidation catalyst is not a steam dehydrogenation; instead, it is carried out in the absence of added steam (though some steam is formed by reaction of the oxygen with hydrogen which is present). In this process of the present invention, we have discovered that oxygen in the absence of added steam is advantageously admixed with the alkane and passed over the catalyst, so that heat produced by the exothermic reaction of the oxygen with hydrogen which is present provides, partially or fully, the heat required by the endothermic dehydrogenation. The hydrogen required for the reaction with the oxygen can be introduced into the reaction zone, but this is not preferred. Advantageously, the hydrogen is hydrogen produced by the dehydrogenation of the alkane to alkene, so as to shift the equilibrium in favour of the alkene. Preferably, the amount of oxygen is such that the dehydrogenation is carried out under adiabatic conditions, so that no heat is supplied (or removed) from the reaction. Especially preferred is the amount of oxygen being such that the endothermic dehydrogenation is balanced by the exothermic reaction of the oxygen with hydrogen which is present so that the temperature remains constant (this situation is referred to herein as thermally neutral conditions). Thus, the optimum temperature for yield, life of catalyst etc can he maintained, eg so that at least 95% selectivity to the alkene is obtained. The amount of oxygen is desirably less than the amount of the alkane, on a molar basis, and preferably less than half the amount of the alkane on this basis. For example, employing isobutane as the alkane, it is preferred that the amount of oxygen be below that indicated by the stoichiometry of the reaction equation: C 4 H 10 + 0.5O 2 → C 4 H 8 + H 2 O. The optimum amount of oxygen will vary with the desired operating temperature, and as a guide we would predict that the maximum amount of oxygen for highly selective, thermally neutral, dehydrogenation of isobutane be 5% at 450°C, 7.5% at 500°C and 9% at 550°C, based on the combined volumes of isobutane and oxygen. The present oxidative dehydrogenation is usually carried out at a temperature from 350 to 550°C, for instance at a temperature from 350 to 480°C, for example when the platinum group metal comprises platinum and the support comprises alumina. The oxidative dehydrogenation is preferably carried out under relatively high space velocities, such as an alkane, and especially a total, gas hourly space velocity (GHSV) of 1000 to 5000hr -1 , for example for isobutane. The operating pressure is conveniently atmospheric, but the dehydrogenation can be operated at above or below atmospheric pressure. If desired, diluent gases can be used, although hydrogen is not recommended as explained above; in addition, it would be an added process cost. The alkane which is dehydrogenated is preferably raw material, not alkane which has already been partially dehydrogenated. The oxygen can be employed as such but conveniently it is employed as a component of an oxygen-containing gas, particularly air. The platinum group metal (ruthenium, rhodium, palladium, osmium, iridium and platinum) of the dehydrogenation and oxidation catalyst is preferably platinum. The catalyst preferably contains 0.1 to 3% by weight of the platinum group metal, eg platinum. The support can be for example alumina, silica, magnesia, titania, zirconia or a mixture or joint oxide ( eg an alumina silicate) thereof, or a Group IIA or IIB ( eg zinc) aluminate spinel. Groups IIA and IIB are as given in the inside front cover of the CRC Handbook of Chemistry and Physics, 60th edition, CRC Press, 1980. Commonly, the support comprises ( ie consists of or includes) alumina. For instance, the catalyst contains as support 10-99.9% by weight of alumina. Promoters can be employed with the platinum group metal. Preferred as promoter is tin oxide. The promoter, when present, is usually employed as 0.1-5% by weight of the catalyst. The catalyst can be obtained in conventional ways, for example by impregnating the support with a precursor of the platinum group metal and a precursor of any co-promoter, and calcining. A particularly advantageous catalyst for the present oxidative dehydrogenation of an alkane in the absence of added steam comprises platinum deposited upon a support which is a mixture of tin oxide and zirconium oxide. The catalyst contains a catalytically effective amount of the platinum. Usually the catalyst contains 0.1 to 3% by weight of platinum, calculated as metal. Additional catalytically active components can be present, though preferably the catalytically active component consists essentially of platinum. The catalyst contains a supporting amount of the mixture of tin oxide and zirconium oxide. Additional support components can be present. It is preferred that the support consists essentially of the mixture of tin oxide and zirconium oxide. Usually the catalyst contains 6-60, preferably 10-60, especially 15-30, % by weight of the tin oxide (measured as tin oxide). Usually the catalyst contains 37-94.9, preferably 70-85, % by weight of the zirconium oxide. The weight ratio of the tin oxide to the zirconium oxide is preferably 1:3-9, especially 1:3-5. In a preferred embodiment, the catalyst comprises 0.1 to 3% by weight of platinum, calculated as metal, 10 to 60% by weight of tin oxide, the balance being zirconium oxide. A particular catalyst has a support comprising SnO 2 and ZrO 2 in a weight ratio of approximately 1:4. One preferred embodiment of the catalyst is prepared by impregnating 1% (by weight, calculated as metal) of a platinum salt or compound onto a co-precipitate of SnO 2 and ZrO 2 in a weight ratio of 1:4. Said catalyst may comprise in addition other components such as promoters and/or stabilisers. The catalyst may be in the form of pellets or other shapes, for example produced by pelletisation or extrusion, or may be supported on high surface area monoliths such as ceramic or metal honeycomb monoliths. The mixture of SnO 2 and ZrO 2 may be formed in a variety of ways and there may be a chemical interaction or compound formation between the components which is as yet not fully understood. The preferred method of preparation is by co-precipitation; suitably by adding NaOH to a mixture of tin and zirconium salts in aqueous solution. The mixture may then be dried and calcined, especially to yield a powdered material with moderately high surface area (typically 95m 2 g -1 ) and narrow pore-size distribution (most of the pores having a radius of about 2nm), before impregnation with an aqueous solution of a platinum salt. The impregnated catalyst is suitably dried and calcined again. In preferred embodiments, the process of the invention: i) provides heat within the catalyst bed by reacting exothermically with some of the hydrogen being formed; ii) by consuming hydrogen, can shift the equilibrium in favour of the desired products; and iii) suppresses two of the major causes of catalyst deactivation, that is over-reduction of the catalyst and carbon deposition. The concentration of oxygen should be carefully controlled at adiabatic conditions, and it is believed that the amount of oxygen should be maintained below stoichiometric relative to the amount of hydrogen present. There are two primary reasons for requiring that the amount of oxygen be carefully controlled, firstly to avoid unwanted products being produced, either from partial or deep oxidation, and secondly to prevent excessive temperature excursions caused by large exotherms. Although the present invention is described herein with particular reference to the oxidative dehydrogenation of isobutane, the invention should not be considered as limited thereto, and may be applied to alkanes in general. It is believed that the greatest benefits of the novel catalyst arise in oxidative dehydrogenation. The alkane is usually of 2-15, preferably 2-5, particularly 3 or 4, carbon atoms. The alkane can be linear, though preferably it is branched. The invention is illustrated by the accompanying drawings, which are two graphs showing yield data and which are described in the Examples hereafter. The invention will now be described with reference to the following Examples. In the prior art, the Pt and Sn are usually supported on Al 2 O 3 , with the loading of Sn being ≤ 5% measured as tin (see J C Hayes, US 4003852). Although there are some references to the use of ZrO 2 (E Clippinger and B F Mulaskey, US 3864284; G J Antos, US 4003826; J C Hayes, US 4003852), its function has been claimed to be simply that of a physical support. EXAMPLE 1 AND COMPARATIVE EXAMPLE 1 Pt-Sn/Al 2 O 3 is a known hydrocarbon-conversion catalyst, which is effective for reactions such as reforming (see T-H Chao et al US 5128300) and direct dehydrogenation (see J W Jenkins, US 3511888) of C 2 -C 20 alkanes. A catalyst with the nominal composition (by mass) of 1%Pt-1%Sn/Al 2 O 3 was prepared (following the method described by F C Wilhelm, US 3998900) by co-impregnating γ-Al 2 O 3 with an aqueous mixture of hydrogen hexachloroplatinate(IV) (chloroplatinic acid) and acidified tin(II) chloride. The resultant material was dried (110°C; air; 24hr) and calcined (500°C; air; 2hr). As is conventional, small amounts of tin oxide are measured and written as Sn and larger amounts, eg 10%, are measured and written as SnO 2 . A packed bed (1cm 3 ) of powdered sample (<150µm particle diameter) was tested in an adiabatic reactor. For measurements of direct-dehydrogenation activity at 450°C, in Comparative Example 1, a gas-feed of undiluted isobutane was used at a flow-rate of 50cm 3 min -1 (GHSV = 3000hr -1 ; MHSV = 6dm 3 hr -1 g cat -1 ). The molar conversion (% isobutane converted to all products) and selectivity (number of moles of isobutane converted to isobutene divided by number of moles of isobutane converted to all products) were recorded as a function of time at selected furnace/gas-inlet temperatures; the molar yield was calculated from the relationship: Yield / % = conversion / % x selectivity / % 100 Oxidative dehydrogenation was carried out, in Example 1, by adding just enough air to the gas-feed to ensure thermally-neutral operation (ie, bed temperature = furnace/gas-inlet temperature ). The space velocity of the isobutane was the same, therefore, as during direct dehydrogenation. Again, the molar conversion and selectivity were recorded as a function of time. In both modes of operation (direct dehydrogenation and oxidative dehydrogenation), the catalyst showed very high selectivity (≥95%) toward isobutene formation. Only during the first 5 minutes of testing was there any sign of an undesired cracking product (propene). In the oxidative mode, the amount of CO 2 formed was just above the detection limit of the GC analyser, no CO was detected. As shown in Table 1, the direct-dehydrogenation activity declined noticeably during the first 60 minutes; thereafter, the de-activation was very gradual. The initial loss in activity coincided with the bed temperature decreasing to a new stable value, as the endothermic reaction reached steady-state. ( Comparative Example 1 ) Direct Dehydrogenation over 1%Pt-1%Sn/Al 2 O 3 Elapsed Time / min Ispbutene Yield / % 450°C 2 16.3 25 15.4 55 15.2 120 15.1 300 14.7 1800 When the catalyst was tested in the oxidative mode, at 450°C ad 500°C, the reaction became thermally neutral when the oxygen concentration reached ca 3 or 4% at 450°C, and ca 5.5% at 500°C. The initial activity at 450°C was higher than for direct dehydrogenation (compare Tables 1 and 2). The isobutene yield is higher at 500°C than at 450°C. (Example 1) Oxidative dehydrogenation over 1%Pt-1%Sn/Al 2 O 3 Elapsed Time / min Isobutene Yield / % 450°C 500°C 2 24.6 27.8 25 18.7 24.6 55 18.0 23.0 100 --- 21.6 180 16.4 19.2 300 15.5 16.5 400 14.9 --- EXAMPLE 2 The catalyst described in Example 1 and Comparative Example 1 was used to dehydrogenate isobutane at 450°C, under the conditions described in Comparative Example 1. The yield of isobutene was allowed to decline to 15%, before air was added to the gas-feed. The activity of the catalyst was then measured as a function of gas-feed composition (Table 3). Oxidative dehydrogenation of isobutane over 1%Pt-1%Sn/Al 2 O 3 at 450°C % Air in Gas-Feed Bed Temperature °C Isobutane Conversion % Isobutene Selectivity % 70 498 24.0 70 65 488 21.8 78 55 478 20.0 84 45 468 20.8 85 35 460 19.5 90 20 450 18.0 95 At high air concentrations, the catalyst bed temperature exceeded the furnace temperature and the major products were isobutene and carbon dioxide. As the concentration of air was lowered, the bed temperature decreased and the selectivity to isobutene improved. An optimum gas-composition was eventually achieved, which resulted in the bed temperature remaining at 450°C, whilst very little carbon dioxide was formed. EXAMPLE 3 1%Pt-1%Sn/ZrO 2 (nominal composition, by mass) was prepared by the method described in Example 1 and Comparative Example 1, except that zirconia was substituted for γ-Al 2 O 3 . The catalyst was subjected to the same tests (at isobutane-GHSV = 3000hr -1 ) as described in Example 1 and Comparative Example 1, except that both oxidative and direct dehydrogenation were carried out at 500°C as well as 450°C. Direct Dehydrogenation Over 1%Pt-1%Sn/ZrO 2 Elapsed time / min Isobutene Yield / % 450°C 500°C 2 15.7 29.2 25 15.2 25.7 60 15.1 25.4 240 14.5 23.6 500 --- 22.4 1150 13.4 --- 1380 13.4 19.0 1500 13.4 18.6 The marked improvement at 500°C can be seen. In the oxidative mode, there was again a marked improvement at 500°C compared to use at 450°C. At 500°C, the zirconia-containing catalyst showed improved durability over the 1%Pt-1%Sn/Al 2 O 3 catalyst (Tables 2 and 5), with the isobutene yield exceeding 20% during the first 185 min of testing (compared to 150 min for Pt-Sn/Al 2 O 3 ). Oxidative Dehydrogenation Over 1%Pt-1%Sn/ZrO 2 Elapsed Time / min Isobutene Yield / % 450°C 500°C 2 19.6 31.6 25 18.3 29.0 55 17.8 26.5 120 15.2 23.1 180 14.6 20.0 300 14.0 18.0 EXAMPLE 4 AND COMPARATIVE EXAMPLE 2 1%Pt-1%Sn/10%ZrO 2 -Al 2 O 3 (nominal composition, by mass) was prepared by the method described in Example 1 and Comparative Example 1, except that ZrO 2 -Al 2 O 3 was substituted for Al 2 O 3 . The mixed-oxide was made by impregnating γ-Al 2 O 3 with an aqueous solution of zirconium nitrate, before drying (110°C; air; 24hr) and calcining (500°C; air; 2hr). The good durability gained by using ZrO 2 (in Example 3) was lost by adding Al 2 O 3 . During oxidative testing (Example 4) at 500°C under identical conditions to those of Example 3, the yield of isobutene dropped from 30.0% to 15.0% in the course of the first 85 min. This shows the deleterious effect of alumina; the catalyst does not have the present supporting amount of the mixture of tin oxide and zirconium oxide. EXAMPLE 5 1%Pt/10%SnO 2 -ZrO 2 (nominal composition, by mass) was prepared by co-precipitating SnO 2 and ZrO 2 from an aqueous mixture of tin(IV) chloride and hydrated zirconium oxychloride, using aqueous sodium hydroxide as the precipitant. The precipitate was washed thoroughly, before being dried (110°C; air, 24hr) and calcined (500°C; air; 2hr). The resultant mixed-oxide was impregnated with aqueous dinitrodiammine platinum(II), before the above drying and calcination steps were repeated. The catalyst was tested under the conditions described in Example 3. The increased loading of Sn was not beneficial to direct dehydrogenation, but resulted in improved durability during oxidative dehydrogenation (compare Tables 5 and 6). At 500°C, the isobutene yield exceeded 20% during the first 210 minutes (compared to 185 minutes for 1%Pt-1%Sn/ZrO 2 ). Oxidative Dehydrogenation Over 1%Pt/10%SnO 2 -ZrO 2 Elapsed Time / min Isobutene Yield / % 450°C 500°C 2 17.1 28.2 25 18.4 28.7 55 18.2 27.6 120 16.5 24.2 180 14.8 21.3 240 --- 18.7 EXAMPLE 6 1%Pt/20%SnO 2 -ZrO 2 (nominal composition, by mass) was prepared by the method described in Example 5, and tested under the conditions described in Example 3. Again, the clearest benefit derived from the high tin loading was apparent in the oxidative mode, both at 450°C and 500°C, when the rate of de-activation was even further reduced (compare Tables 5, 6 and 7). In particular, the durability at 450°C (as measured by the duration of yield ≥ 15%) exceeded 24hr (compared to 6hr for 1% Pt-1%Sn/Al 2 O 3 ); see Figure 1. Oxidative dehydrogenation over 1%Pt/20%SnO 2 -ZrO 2 Elapsed Time / min Isobutene Yield / % 450°C 500°C 2 19.0 38.7 25 18.8 31.4 55 18.6 29.0 180 18.0 24.0 300 17.6 20.8 400 --- 18.8 1260 15.1 --- 1500 14.9 --- EXAMPLE 7 A fresh sample of 1%Pt/20%SnO 2 -ZrO 2 (as described in Example 6) was treated under oxidative conditions, but at half the space velocity used in Examples 1-6 and Comparative Examples 1 and 2. At this lower space velocity (isobutane-GHSV = 1500hr -1 ), the catalyst de-activated more gradually. As Figure 2 shows, its initial activity at 500°C was similar to that observed in Example 6, but the yield still exceeded 25% after 5 hours (the time taken for the yield to fall below 20% at isobutane-GHSV = 3000hr -1 ). EXAMPLE 8 The ability of 1%Pt/20%SnO,-ZrO 2 (as described in Example 6) to dehydrogenate linear alkanes was tested by following the procedures given in Example 3, but replacing the isobutane reactant with normal butane. During direct dehydrogenation, the initial total yield of unsaturated products was 26% (product selectivity: 32% 1-butene, 27% cis 2-butene, 38% trans 2-butene, 2% butadiene), but declined to 14% in the space of 3 hours. On switching to the oxidative mode, without first regenerating the catalyst in any way, the total yield recovered, reaching a maximum of 29% before declining slowly (down to 25% after a further 2 hours); the product distribution was very similar to that observed during direct dehydrogenation. EXAMPLE 9 The sequence of tests described in Example 8 was repeated using propane as the alkane reactant. During direct dehydrogenation, the initial yield of propene was 19%; after 3 hours, it had declined to 12% On switching to the oxidative mode, the yield was restored to 19%. Thereafter, it declined slowly to 17% during the next 4 hours. The invention is further illustrated by the yield data given graphically for various catalysts in the accompanying Figures 1 and 2. FIGURE 1 : Oxidative dehydrogenation of isobutane (GHSV = 3000hr -1 ) at 450°C, over (a) 1%Pt/20%SnO 2 -ZrO 2 ; (b) 1%Pt-1%Sn/Al 2 O 3 . FIGURE 2 : Oxidative dehydrogenation of isobutane at 500°C, over (a) 1%Pt/20%SnO 2 -ZrO 2 ; (b) 1%Pt/20%SnO 2 -ZrO 2 ; (c) 1%Pt-1%Sn/ZrO 2 ; (d) 1%Pt-1%Sn/Al 2 O 3 . For (a), GHSV = 1500hr -1 ; (b) - (d); GHSV = 3000hr -1

Claims: A process for the dehydrogenation of an alkane to form an alkene, comprising passing a feedstock comprising said alkane in the gas phase in admixture with oxygen and in the absence of added steam over a dehydrogenation and oxidation catalyst comprising a platinum group metal deposited upon a support. A process according to claim 1, wherein the amount of oxygen is such that the endothermic dehydrogenation is balanced by the exothermic reaction of the oxygen with hydrogen which is present so that the temperature remains constant. A process according to claim 1 or 2, wherein the catalyst contains 0.1-3% by weight of platinum. A process according to any one of claims 1-3, wherein the catalyst contains also 0.1-5% by weight of tin oxide. A process according to any one of the preceding claims, wherein the catalyst contains as support 10-99.9% by weight of alumina. A process according to any one of the preceding claims, wherein the alkane is isobutane. A process according to any one of the preceding claims, wherein the dehydrogenation is carried out at a temperature from 350 to 480°C.

Inventors: GOLUNSKI STANISLAW EDMUND, HAYES JOHN WILLIAM, GOLUNSKI, STANISLAW EDMUND, HAYES, JOHN WILLIAM
Applicants: JOHNSON MATTHEY PLC, JOHNSON MATTHEY PUBLIC LIMITED COMPANY
Application year: 1994
Main classification: C07C11
IPC codes: B01J23, B01J21, C07C11, C07C5, C07B61, B01J37, C07B35